This methods ensures n_e(ψ) is everywhere positive and C²-continuous.
The density boundary is now dependent on the width w of the mollifier,
which also controls the smoothness of the curve, specifically:
ψ_bnd = ψ_last + w
where ψ_last = 1.01 currently is an extra point added to improve the
convergence of the mollified density to the original data as w→0.
While technically accepted by GRAY, these indices do not carry a special
meaning, as wrongly implied by the documentation: they are equivalent
to 8, 18 and specifically don't change the meaning of sgnbi,sgni.
- Documentation is not built anymore with the default `all` rule to
improve portability. It must be built explicitly with `make docs`.
- Font types are not specified to allow building on systems with a
restricted set of fonts.
- Syntax fixes in the documentation Markdown.
